The present invention relates to a method for processing a signal from a flow meter for measuring a gas flow in an internal combustion engine so as to determine the quantity of air taken into the engine. The invention also relates to a method for configuring a system for processing a signal from a flow meter for measuring a gas flow in an internal combustion engine. The invention also relates to a data medium comprising computer programs for governing these methods. The invention further relates to a processing system, a measuring system comprising such a processing system and a motor vehicle comprising such a processing system or such a measuring system.
The future anti-pollution standards will require motor vehicle manufacturers to improve engine monitoring, that is to say, to control the engine more finely so that its impact on the environment is reduced. For this, it is necessary to accurately know the quantities of air supplied to the combustion chambers of the engine. To this end, air flow meters are used to measure the air flow rate feeding the engine. The engine may be any kind of combustion heat engine, in particular internal combustion heat engines with no distinction concerning the numbering of strokes in the operating cycle, the injection mode, etc. The invention thus applies:                to diesel engines, with or without supercharging, any combustion mode,        to controlled ignition petrol engines, with or without supercharging, any combustion mode,        to flex-fuel engines using a variable mixture of petrol and ethanol,        to gas engines.        
On a combustion engine, any flow rate information (by volume or by mass) is likely to be measured by virtue of a flow rate sensor such as a flow meter or a pressure sensor. The raw signals from a flow rate sensor are processed by a computer program contained in a computer or an electronic control unit, the computer program making it possible to condition the signals and correct them, notably to filter them, in order to obtain reliable information that can be used by engine control strategies. Before these processing operations, a mapping is usually used to convert the electrical signals into physical flow rate information.
The principle of flow rate measurement by a flow meter is based on a measurement of local speed in a section of the flow meter, then a multiplication of the measured speed value by the area of the section of the flow meter. However, this flow rate measurement principle is sensitive to 2 factors:                the speed profile in the section, this speed not being constant because of aerodynamic phenomena,        the flow rate pulsings that result from the cyclic operation of the engine.        
It also appears that the speed of rotation of the engine and the load are two main variables that influence the form of the flow rate pulsing as shown in FIG. 1, in which 4 curves are represented giving the trends over time of gas speed in a nozzle of the engine for engine rotation speeds of 800 revolutions per minute, 1400 revolutions per minute, 1700 revolutions per minute and 2000 revolutions per minute, all these curves being related to the same 5-bar load.
It is possible, for obtaining an accurate flow rate value, to use a mapping that is a function of the load and speed of the engine. The processing subsystem for processing the signal delivered by a flow meter is therefore described below with reference to FIG. 2.
A flow meter 11 supplies an electrical signal that has a frequency or a voltage dependent on the gas flow rate passing through the flow meter. Thus, the frequency of the signal is an image of the gas flow rate. A period counter 12 discretizes this frequency, so a discrete frequency is thus obtained at the output of the counter 12. By virtue of a linearization means 13, the discrete frequency is then transformed into a discrete flow rate. This linearization means uses a linearization curve for the flow meter representing the gas flow rate values passing through the flow meter as a function of the frequency of the flow meter output signal. The discrete air flow rate remains an instantaneous item of information which is then averaged over a half-revolution of the engine by virtue of a means 14, then filtered by a first-order filter 15 to give an average gas flow rate information item at the output of the filter 15. Finally, this average air flow rate is corrected by an operator 16 using, for this correction, one or more values supplied by a mapping 17. This mapping 17 supplies one or more values as a function of an engine load value and an engine rotation speed value. Thus, it is possible to obtain an accurate gas flow rate value regardless of the engine load torque and rotation speed values.
Nevertheless, for the future, and from today, it is planned to supplement the intake circuits for the gases used in the operation of the engine with various actuators, notably valves and/or dampers, which, depending on their state, define different intake modes for the engine or different engine intake configurations.
From now on, the intake mode of the engine is no longer defined solely by the engine rotation speed and engine load parameters, but also by the states of these actuators. For example, in the case of future diesel engine production projects compliant with the Euro 6 standard, the engines will have two intake modes:                a mode A in which an exhaust gas recirculation EGR takes place in a high-pressure section of the intake circuit,        a mode B in which an exhaust gas recirculation EGR takes place in a low-pressure section of the intake circuit.        
Hitherto, if the processing subsystem described previously is used after having configured it so that it is designed to supply accurate gas flow rate values when the engine is operating in mode A, the graph represented in FIG. 3 will be obtained. It should be noted that, when the engine is operating in mode A, regardless of the air flow rate taken into the engine, the measurement error on the flow rate value supplied by the flow meter is within a range of +/−3%.
On the other hand, it should be noted that, when the engine is operating in mode B, the measurement error on the flow rate value supplied by the flow meter is outside the range of +/−3% and even reaches +/−10%, as represented in FIG. 4.
The error range of +/−3% represents the acceptable limit of the measurement error on the flow rate for the Euro 6 diesel engine projects.
The various error data from the preceding graphs are obtained by calculating the difference between the flow rates measured by virtue of the flow meter and supplying information by virtue of the processing subsystem defined previously and flow rates measured by virtue of a reference flow meter that is insensitive to the intake modes of the engine for example, this reference flow meter uses an analysis of the composition of the exhaust gases from the engine.
Similarly, the processing subsystem could be configured and adapted to supply accurate flow rate value when the engine is operating in intake mode B. In this case, it would be the flow rate values measured when the engine is operating in mode A that would not have the required accuracy.